At present, many hybrid automobiles, electric automobiles, or fuel battery automobiles incorporate a secondary battery as an electric power supply for supplying electric power required to start the engine and drive the vehicle.
The secondary battery keeps degraded depending on the ambient environment and the way in which it is used. Generally, when the secondary battery is degraded, the internal impedance thereof increases, and the capacity thereof decreases.
When the internal impedance increases, the supply of electric power from the secondary battery is restrained. Therefore, a vehicle which uses a secondary battery as an electric power supply suffers a performance change. It is therefore necessary to detect a degraded state of the secondary battery and optimally control the vehicle based on the detected degraded state. If the secondary battery is degraded to the point where the degradation possibly causes serious trouble in the vehicle performance, then the secondary battery needs to be replaced.
Generally, the connection between secondary battery 100 and load 200 can be represented by a model shown in FIG. 1. In FIG. 1, secondary battery 100 has OCV (open circuit voltage) 101 and Z (internal impedance) 102. Voltage V is the voltage of secondary battery 100, and current I is a current flowing through secondary battery 100.
If the value of OCV 101 is known, then the internal impedance of secondary battery 100 can be determined according to the equation Z=(OCV−V)/I. However, the value of OCV 101 is often uncertain.
Even if the value of OCV 101 is not known, the internal impedance of secondary battery 100 can be determined according to the equation Z=ΔV/ΔI. ΔI represents a change in the current in a short time, and ΔV represents a change in the voltage at the time (see JP-A No. 10-214643).
JP-A No. 2000-121710 discloses a method of determining degradation of a secondary battery. Specifically, in order to measure the internal impedance of a secondary battery, a charging and discharging current is caused to flow in a predetermined pattern, and a change in the voltage at the time is measured. The internal impedance of the secondary battery, is calculated based on the change in the voltage. Then, degradation of the secondary cell is judged based on the calculated result.
Patent Document 1: JP-A No. 10-214643
Patent Document 2: JP-A No. 2000-121710